Sounds like fear of commitment and real intimacy. You dont have to truly commit to someone who is already taken so you pursue people in relationships. Is this an ego boost to you? Do you for lack of a better word, "get off" on knowing that you were able to seduce someone who is married?

Do you have abandonment issues? mom or dad bail on you emotionally, physically as a kid? some things to think about. Please stop pursuing people in relationships, you cant feel good about yourself when doing this. Good luck.